This invention relates to a manufacturing process for a semiconductor wafer (hereinafter also referred to as a xe2x80x9cwaferxe2x80x9d) including surface grinding, followed by planarization or polishing.
In a manufacturing process for a semiconductor wafer of this kind, a semiconductor wafer has been surface-ground according to a process described below, shown in FIG. 10(a)(b)(c) and thereafter, a single side surface or double side surfaces thereof are processed for planarization according to a CMP (Chemical Mechanical Polishing) method, or a PACE (Plasma Assisted Chemical Etching) method (for example, see JP 2565617) for purposes of removal of a work damage layer caused by the grinding, and improvements on flatness and smoothness.
(1) First, a chuck table 12 for fixing a semiconductor wafer W thereon rotates on its rotation axis 12a with a prescribed slight tilt angle xcex8 to the rotation axis 14a of a rotary shaft 14 of a cup shaped grinding wheel (hereinafter also simply referred to as a xe2x80x9cgrinding wheelxe2x80x9d) 16. Then, the grinding wheel 16 in rotation moves down and is into contact with the chuck table 12 to grind the chuck table 12 (FIG. 10(a)), with the result that a grinding finished surface of the chuck table 12 assumes an outwardly extending circular cone surface having a vertical angle (180xe2x88x922xcex8) with a vertex thereof being the rotation axis of the chuck table 12.
(2) Then, a semiconductor wafer W is fixed on the surface ground chuck table 12 and the chuck table 12 is forced to rotate (FIG. 10(b)).
(3) Furthermore, the grinding wheel 16 moves down to a surface of the semiconductor wafer W and is into contact with the 40 wafer W, thereby the wafer W being surface-ground (FIG. 10(c)).
In recent years, a trend that surface grinding, as a processing method to obtain higher flatness, is introduced into a wafer processing step has been increasingly enhanced due to the demand for higher flatness of a semiconductor wafer. In the surface-grinding step of the semiconductor wafer, as shown in FIG. 10(a) to (c), the grinding is performed such that the cup shaped grinding wheel 16 cuts into the wafer W at an outer peripheral edge thereof and moves away from the wafer at a central portion, wherein the grinding proceeds while the grinding wheel 16 keeps in contact with the wafer over a length corresponding to a radius of the wafer W.
As schematically shown in FIG. 11, a removal by grinding is low due to a surface slipping phenomenon from elasticity of the grinding wheel 16 at a start position for cutting in of the grinding wheel 16 in contacting the wafer as a work and as a result, a grinding finished surface comes to have a slight rise at the start position, while at terminating position for cutting, the grinding finished surface face comes to have a recess because a grinding removal increases due to rapid reduction in resistance to cutting. These rise and recess characteristic of a,finished surface obtained by surface-grinding have been obstacles against manufacturing a wafer having higher flatness.
For example, in the case where surface-grinding is effected using a resinoid bonded cup shaped grinding wheel including diamond abrasive powder and furthermore, the grinding is performed toward the center of the-wafer from an outer peripheral edge thereof, as was in the prior art, such that the grinding wheel cuts into the wafer at the outer peripheral edge thereof and moves away from the wafer at the center thereof, there occur a recess in the vicinity of the center of the surface-ground wafer, of the order of 2 to 10 mm in diameter and of the order of 0.2 to 0.5 xcexcm in depth, and a rise of the same order as the recess in size in the outer peripheral edge portion thereof.
The surface-ground wafer proceeds into a planarization or polishing step to come next, where a work damage layer on the surface-ground surface of the wafer caused by the surface-grinding is removed and the above described rise and recess portions are corrected to achieve increased flatness and enhanced smoothness.
In a polishing method using a polishing cloth, that is a CMP method, which is currently a main stream as a planarization or polishing method, the polishing cloth is made of a soft material, therefore, there is a tendency that a polishing surface of the cloth follows geometric features of a to-be-polished surface of a work and as a result, a polishing action is exercised to even a recess portion in the vicinity of the center of the wafer, therefore, correction of the recess portion is hard to be effected. In contrast to this, a polishing action is easy to act at a rise portion occurring locally in the outer peripheral edge portion, therefore, a rise in the outer peripheral edge portion is corrected.
On the other hand, in a PACE method newly developed for planarization, stock can be removed from a to-be-ground surface locally to a prescribed depth by an etching action of a plasma; therefore, a surface material is removed from part other than the recess portion, whereby flatness all over the surface can be improved.
The invention has been made in light of the above described prior art problem and it is accordingly an object of the invention to provide a manufacturing process for a semiconductor wafer capable of manufacturing the semiconductor wafer having higher flatness efficiently from a wafer work having passed through a surface grinding step by suppressing the above-noted reductions in flatness occurring in the vicinity of the center of the wafer work and in the outer peripheral edge portion thereof as much as possible and further, enabling correction of the reductions in flatness for planarization in a planarization or polishing step with ease.
In order to solve the above problem, a first aspect of the invention is directed to a manufacturing process for a semiconductor wafer wherein when the semiconductor wafer is surface ground using a cup shaped grinding wheel, the semiconductor wafer is ground toward the center thereof such that the cup shaped grinding wheel cuts into the semiconductor wafer at an outer peripheral edge thereof and moves away from the semiconductor wafer at a central portion thereof and the ground semiconductor wafer is planarized according to a PACE method.
That is, since in a surface grinding operation, the grinding wheel cuts into the semiconductor wafer at the outer peripheral edge portion and moves away from the semiconductor wafer at the central portion, there occur a rise in the outer peripheral edge portion of a finish surface of the wafer after the grinding and a recess in the central portion thereof. However, by use of the PACE method, there is corrected the recess in the central portion of the wafer, which in the prior art was difficult to be corrected by means of a method for polishing the wafer using a CMP method after the surface grinding, thereby enabling manufacture of a polished wafer having higher flatness.
A surface of the semiconductor wafer which has been planarized using the PACE method has very fine rises and recesses thereon remaining after the planarization; therefore, it is desirable to improve flatness and smoothness of the surface by further polishing the surface using a CMP method.
A method to force the grinding wheel to cut into the semiconductor wafer at the outer peripheral edge thereof can be performed such that rotation directions of the semiconductor wafer and the grinding wheel can be either of the same sense as each other or of the opposite senses from each other.
A chuck table, as described above, has a finished surface of a slightly inclined circular cone whose vertex is on the rotation axis of the chuck table. The wafer is held on the chuck table and surface grinding is performed so as to act over a radius of the wafer surface while holding a state of the wafer in which a central portion of a to-be-ground surface of the wafer slightly protrudes from a plane including a grinding surface of the grinding wheel; therefore, an angle a formed between the rotation axis of the rotary shaft of the grinding wheel and the rotation axis of the chuck table is necessary to be adjusted according to whether rotation directions of the semiconductor wafer, that is the chuck table, and the grinding wheel are the same sense as each other, or the opposite sense from each other.
That is, as shown in FIG. 5, in the case where rotation directions of the grinding wheel and the chuck table are the same sense as each other, the rotation axis of the chuck table has to be inclined by xcex8 along a clockwise direction, as shown in the figure, to the rotation axis of the rotary shaft of the grinding wheel, while to the contrary, in the case of FIG. 6 where rotation directions of the grinding wheel and the chuck table are opposite from each other, the rotation axis of the chuck table has to be inclined by xcex8 along a counterclockwise direction to the rotation axis of the rotary shaft of the grinding wheel, whereby grinding can be performed such that the grinding wheel is cut into the wafer at the outer peripheral edge portion and moves away from the wafer at the central portion.
A second aspect of the present invention is directed to a manufacturing process for a semiconductor wafer wherein when the semiconductor wafer is surface ground using a cup shaped grinding wheel, the semiconductor wafer is ground toward the outer periphery thereof such that the cup shaped grinding wheel cuts into the semiconductor wafer at the center thereof and moves away from the semiconductor wafer at an outer peripheral edge thereof and the thus ground semiconductor wafer is planarized by means of a CMP method.
That is, in this case, since when the semiconductor wafer is surface ground, the grinding wheel cuts into the semiconductor wafer at the center thereof and moves away from the semiconductor wafer at an outer peripheral edge portion thereof, there occur a rise in the central portion of the grinding finished surface of the wafer and sag in the outer peripheral edge portion. Hence, in this case, since the rise in the central portion can be corrected by a polishing action of the CMP method with ease, the prior art problem is solved and thus a wafer having high flatness all over the polishing finished surface thereof can be manufactured. Note that since the wafer rotates about a rotation axis passing substantially through the center of a ground surface being held by a chuck, a peripheral velocity due to rotation thereof is larger at a point on the ground surface closer to the outer periphery; therefore, a contact time of the ground surface of the wafer per unit area of the surface with the grinding wheel in the outer peripheral edge portion is shorter and accordingly an edge sag in the outer peripheral edge portion due to grinding is relatively small. Since in a CMP method the surface of a polishing cloth deforms in conformity with geometrical features of the ground surface of the wafer, an edge sag in the outer peripheral edge portion of the polished surface of the wafer has almost no difference in dimension from that under a polishing action of a normal CMP method.
A method to force the grinding wheel to cut into the semiconductor wafer at the center thereof can be performed such that rotation directions of the semiconductor wafer and the grinding wheel can be either of the same sense as each other or of the opposite senses from each other.
In order that the grinding wheel cuts into the wafer at the center thereof and moves away from the wafer at the outer peripheral portion thereof, it is necessary to adjust an angle xcex8 of the rotation axis of the chuck table to the rotation axis of the rotary shaft of the grinding wheel according to whether rotation directions of the chuck table and the grinding wheel is of the same sense or of the opposite senses. The purpose can be achieved such that in the case of the same sense the rotation axis of the chuck table is, as shown in FIG. 7, inclined counterclockwise by xcex8 relative to the rotation axis of the rotary shaft of the grinding wheel, while on the other hand, in the case of the opposite senses the rotation axis of the chuck table is, as shown in FIG. 8, inclined clockwise by xcex8 relative to the rotation axis of the rotary shaft of the grinding wheel.
Moreover, it has been found that in order that, as is in a method of the invention, a wafer with extremely high flatness is manufactured with good efficiency by combining surface grinding with a planarization or polishing method following the surface grinding, there is a preferable combination between a cutting-in position on the chuck table by the grinding wheel when a chuck surface of the chuck table is surface ground in advance of surface grinding of the wafer and a cutting-in position on the wafer of the grinding wheel when the wafer is surface ground.
That is, when the chuck surface is ground to slightly form a circular cone surface by surface grinding of the chuck table in advance on which the semiconductor wafer is fixed as well, there occur a small rise in the vicinity of a cutting-in position of the grinding wheel on the chuck surface after the surface grinding and a recess in the vicinity of a position at which the grinding wheel moves away from the chuck table. Therefore, when the grinding wheel cuts into the wafer at the outer peripheral edge portion and moves away from the wafer at the central portion thereof in a surface grinding operation of the wafer, it is preferable that in a surface grinding operation of the chuck table, the grinding wheel, likewise, cuts into the chuck table at the outer peripheral edge portion thereof and moves away therefrom at the central portion thereof (FIG. 12(a)).
That is, in this case, as shown in FIG. 12(b), there occur a small rise on a circular cone surface in the outer peripheral edge portion of the chuck table and a recess in the vicinity of the vertex of the circular cone, which is the center of the chuck table; therefore, when a wafer is fixed on such a chuck table, a to-be-ground surface of the wafer comes to have a recess corresponding to that in the central portion of the chuck table, while having a small rise in the outer peripheral edge portion of the to-be-ground surface of the wafer corresponding to the rise in the outer peripheral edge portion of the chuck surface of the chuck table (FIG. 12(c)).
When the wafer held in this state is ground after the grinding wheel cuts into the wafer at the outer peripheral edge portion thereof as described above (FIG. 12(d)), the wafer ends up with grinding finish in a state where there occur a small rise in the outer peripheral edge portion and a recess in the central portion of the wafer as held on the chuck table, with the result that thicknesses in the outer peripheral edge portion and the central portion of the wafer are almost the same as those in the other portions; therefore, the wafer retains a high level of flatness in a state after the wafer is separated from the chuck table (FIG. 12(e)) and correction in a following planarization or polishing step is thus effectively realized.
Contrary to this, in the case where a chuck table is ground similar to FIG. 12(a) (FIG. 13(a)) and there occur a small rise in the peripheral edge portion of the chuck table and a recess in the vicinity of the vertex of a circular cone, which is the center of the chuck table (FIG. 13(b)), a wafer is fixed on the chuck table (FIG. 13(c)), surface grinding of the wafer is performed such that the grinding wheel cuts into the wafer in rotation at the central portion thereof, proceeds toward the outer peripheral edge portion of the wafer in rotation and then, moves away from the wafer at the outer peripheral edge portion (FIG. 13(d)). In this case, a rise is observed in the central portion of the wafer even in a chucked state since a removal is less in the central portion of the wafer, while an edge sag is observed in the outer peripheral edge portion of the wafer even in the chucked state since an excessive removal occurs in the outer peripheral edge portion, with the result that the surface-ground wafer has a sectional profile in which degradation of flatness in the central portion and outer peripheral edge portion, as shown schematically in FIG. 13(e), is further exaggerated up to a state that the following planarization or polishing is unpreferably performed.
This leads as follows. When surface grinding of a wafer is performed such that a grinding wheel, likewise, cuts into the wafer at the central portion thereof and moves away from the wafer at the outer peripheral edge portion thereof (FIG. 14(d)), surface grinding is preferably applied to a chuck surface of the chuck table such that the grinding wheel, likewise, cuts into the chuck table at the central portion thereof and moves away from the chuck table at the outer peripheral edge portion thereof (FIG. 14(a)).
Note that as a cup shaped grinding wheel used in a surface grinding operation on the chuck table, a metal bonded grinding wheel is preferable and as a cup shaped grinding wheel used in a surface grinding operation on the wafer, a resinoid bonded grinding wheel or a vitrified grinding wheel is preferable.
The ground surface finished by surface grinding in such a way as described above has currently inevitably grinding striations thereon caused as shown in FIG. 9 by alterations in a contact state between an acting surface of the grinding wheel and a ground surface of a to-be-ground work in grinding affected by a tiny displacement or vibrations of a drive section in operation of an apparatus, flatness of an acting surface of a grinding wheel, distribution in particulate shape of abrasive powder and others.
In addition, it has been found in the course of development maturing to the invention that shapes of the grinding striations are related with polishing efficiency in CMP polishing performed later and a lifetime of a polishing cloth.
As shown schematically in FIGS. 9(a) and 9(b), each grinding striation is produced in a state that the grinding wheel 16 drawn with a broken line is put into contact with a wafer at a solid line portion thereof and grinds the surface of the wafer. A detailed study was conducted on a relationship between a shape of a grinding striation and grinding conditions providing good working efficiency, with the result that it is preferable that the semiconductor wafer is surface ground such that grinding striations produced on a ground surface of the semiconductor wafer each assumes a convex toward a direction of rotation of the semiconductor wafer in a following polishing step, along a circular arc line starting from the center of the ground surface and having an arbitrary radius.
That is, when grinding striations are produced on the ground surface of a wafer as shown in FIG. 9(a), which rotates by friction between the wafer and the polishing cloth in CMP, the direction of the rotation is preferably in the direction of an arrow as viewed from the to-be-ground surface, while in the case of FIG. 9(b) the rotation direction of the wafer is in the direction of an arrow as well, and it is preferable that the grinding striations each assumes a convex along a circular arc line starting from the center of the wafer and having an arbitrary radius.
In such a way, when grinding striations produced on a semiconductor wafer are each convex toward the rotation direction of the semiconductor wafer in a later coming polishing step, a resistance to polishing is less; therefore, vibrations of the wafer during polishing are on a small scale, leading to an advantage of a longer lifetime of a polishing cloth.
To the contrary, when grinding striations produced on a semiconductor wafer are each concave toward the rotation direction of the semiconductor wafer in a later coming polishing step, a resistance to polishing increases and the wafer vibrates vigorously during the polishing, resulting in an adverse influence on a lifetime of a polishing cloth.